HRT increased the risk of ovarian cancer by 20 per cent, according to the Million Women Study, which involved a quarter of all women aged 50–64 in England.

Previous research from the same study had suggested that HRT could increase the risk of ovarian and breast cancer. The latest findings say it could raise the combined risk of ovarian, endometrial and breast cancer by more than 60 per cent.

Lead researcher Professor Valerie Beral, director of Cancer Research UK’s epidemiology unit at the University of Oxford, said: ‘This study, along with our previous research, clearly demonstrates the cancer risks of taking HRT.’

A separate study linked a decrease in breast cancer rates in the US to a drop in HRT use. Breast cancer rates fell by 6.7 per cent in 2003, coinciding with a decrease in HRT use, which fell from 62 million scrips in 2000 to 24 million scrips in 2003.

However, Cornwall GP Dr Sarah Gray, a member of the British Menopause Society, criticised the studies for producing shocking figures for cancer risk.

‘The Million Women Study is based on extrapolating results and making a lot of assumptions. There is no firm evidence behind the results of this study,’ she said.

‘It has come under a lot of criticism because it is based on observational studies rather than randomised, control-led trials.’

Oxford GP Dr Sally Hope, who has an interest in menopause and HRT, said that the she would continue to prescribe HRT despite the new findings.

The risk of ovarian cancer was small, amounting to only one extra case of ovarian cancer among every 2,500 women receiving HRT, said Dr Hope.

Consultant physician and endocrinologist Dr John Stevenson, chairman of the charity Women’s Health Concern, said: ‘The Million Women Study does not add up. HRT must be saving a lot of women from other causes of mortality since the study looked at 27,000 women and found no increase in the overall death rate.’

When HRT is taken by women in their fifties, the intended age group, there is a significant reduction in the death rate, added Dr Stevenson.